In recent years, as an environmentally friendly vehicle, electrically powered vehicles such as electric cars, hybrid cars and fuel cell cars have been developed into practical use. These electrically powered vehicles have mounted thereon an electric motor generating force to drive the vehicle and a power supply system for supplying electric power to drive the motor, that is configured to include a power storage device.
In particular for hybrid cars, there has been proposed a configuration charging a vehicle-mounted power storage device by a power supply external to the vehicle (hereinafter also referred to as an “external power supply”), and accordingly, these electrically powered vehicles require increased distances travelable on electric power stored in the vehicle-mounted power storage device. Hereinafter, charging a vehicle-mounted power storage device by an external power supply will also simply be referred to as “external charging”.
For example, Japanese Patent Laying-Open No. 2008-109840 (Patent Document 1) and Japanese Patent Laying-Open No. 2003-209969 (Patent Document 2) describe a power supply system having a plurality of power storage devices (batteries) connected in parallel. The power supply system described in Patent Documents 1 and 2 is provided with a voltage converter (a converter) for each power storage device (battery) as a charging/discharging adjustment mechanism. In contrast, Japanese Patent Laying-Open No. 2008-167620 (Patent Document 3) describes a configuration of a power supply device in a vehicle having a main power storage device and a plurality of sub power storage devices mounted thereon, that provides a converter associated with a main power supply device and a converter shared by the plurality of sub power storage devices. This configuration can achieve a reduced number of elements in the device and also an increased storable amount of energy.
Furthermore, Japanese Patent Laying-Open No. 2006-77641 (Patent Document 4) describes a series/parallel hybrid electric car which has mounted thereon a control device for switching the driving state of a vehicle based on the vehicle speed and an SOC. The driving state includes a state in which an output from an engine (internal combustion engine) is used as motive power for traveling and a state in which motive power for traveling is obtained by stopping the engine and operating a motor.    Patent Document 1: Japanese Patent Laying-Open No. 2008-109840    Patent Document 2: Japanese Patent Laying-Open No. 2003-209969    Patent Document 3: Japanese Patent Laying-Open No. 2008-167620    Patent Document 4: Japanese Patent Laying-Open No. 2006-77641